1. Field of the Invention
The present invention relates to a magnetic recording medium in the continuous form which is provided with a back layer.
2. Description of Prior Art
A magnetic recording medium is widely used for recording electromagnetic signals, and various forms such as forms of tape, disc or card are appropriately applied to the medium according to the purpose. A magnetic recording medium in the form of tape (or in the continuous form; also referred hereinafter to simply as "magnetic tape") is employed as an audio tape, a video tape or a recording medium in a computer system.
Among the magnetic tapes, an audio tape or a video tape has been widely used as a tape encased in a cassette. Recently, the demand for long-play recording has increased with respect to the magnetic tape encased in a cassette, and hence a thinner magnetic tape has been employed in order to encase a magnetic tape having much longer length in the standard cassette.
For imparting high mechanical or physical strength to such thin magnetic tape and further accomplishing smooth contact between the back surface of the magnetic tape and the running system of a recording and reproducing device so as to ensure high running properties (i.e., smooth running), a magnetic tape provided with a back layer (or backing layer) on the surface of the nonmagnetic support not facing the magnetic recording layer has been developed and utilized.
The back layer generally comprises a binder and nonmagnetic particles dispersed therein. The back layer is required to have moderately protruded and depressed portions on its surface to enhance running properties of the magnetic tape. When the surface of the back layer is excessively smooth, the area on the surface of the back layer which is in contact with the running system increases to raise the friction coefficient on the surface of the back layer, whereby the running properties of the magnetic tape deteriorate. On the contrary, if excessively protruded and depressed portions are given on the surface of the back layer, the protruded and depressed portions are liable to be transferred onto the magnetic recording layer which is in contact with those portions in the preparation of a magnetic tape, especially when the tape kept in the wound state is subjected to a heat treatment. That is, protrusion and depression are copied on the surface of the magnetic recording layer, and hence the resulting magnetic tape deteriorates in electromagnetic conversion characteristics. Accordingly, it is required that not only is the particle size distribution of the nonmagnetic particles appropriately adjusted but also dispersibility of the nonmagnetic particles in the binder is improved in the preparation of a back layer.
For these reasons, studies have been made concerning influences of the binder on a surface of the back layer, etc. For instance, Japanese Patent Provisional Publication No. 56(1981)-98719 proposes to use a combination of a cellulosic resin, thermoplastic polyurethane elastomer and polyisocyanate as a preferred binder employable for the preparation of a back layer.
While such binder is satisfactory from the viewpoint of a function of the back layer, a careful attention is required to be paid in the preparation of a back layer using such binder, because nitrocellulose, which is a representative cellulosic resin, has an explosive nature, depending upon the treating conditions.
Otherwise, there can be employed a combination of a conventional polyurethane resin and a vinyl chloride resin as the binder, and in this case the employment of nitrocellulose can be avoided. However, since the nonmagnetic particles are poorly dispersed in such binder, the surface of the back layer obtained is likely provided with excessively protruded and depressed portions. Hence the obtained magnetic tape still easily deteriorates in the electromagnetic conversion characteristics. Further, the back layer prepared using the above-mentioned binder has poor mechanical or physical strength, and hence cracks are easily produced on the surface of the back layer, or an edge portion thereof is easily broken.